Ultra high frequency conversion apparatus



Feb. 25, 1947. 2,416,577

ULTRA HIGH FREQUENCY CONVERSION APPARATUS 7 G. w. FYLER Filed Nov. 30, 1942 CATHODE T0 GROUND CAPACITY FREQUENCY Inventor George W. P

ler,

by His Attorneg.

Patented Feb. 25, 1947 ULTRA HIGH FREQUENCY CONVERSION APPARATUS George W. Fyler, Stratford, Conn, assignor' to General Electric Company, a corporation of New York Application November 30, 1942, Serial No. 467,286

invention relates to frequency converterssand local oscillators for such receivers which possess increased signal to noise ratios. a

Generally in radio communication systems the strength of the received signal is well above the signal to noise ratio is doubled, the effect may be the same as a four to one increase of the power of the transmitter.

Ultra high frequencies are normally employed in radio locating and range finding apparatus, and losses become important which are negligible in apparatus operated at lower frequencies. Various losses in the input circuit of the apparatus tend to decrease the signalstrength, but in most cases these losses maybe reduced so that they become negligible. To a greater degree the strength of the signal may be reduced by the input conductance of the electron discharge tube employed in the circuit. The input conductance of an electron discharge tube depend upon several factors, including dielectric losses in the glass envelope, resistance losses in the grid and cath- Ode leads within the envelope, loading effect due to the transit time of electrons through the tube, and losses due to degeneration caused by the self-inductance of the cathode lead, and, in three electrode tubes, by the grid'to plate capacity of the tube. The loading efiects produced by degeneration in triode converters constitute a large portion of the input conductance, and it is one object of my invention to provide an improved high frequency receiving apparatus including an arrangement forreducing the input conductance of the electron discharge device by neutralizing degeneration produced both by cathode lead inductance and by grid to plate capacity.

It is another object of my invention to provide an ultra high frequency receiving apparatus including an improved arrangement for minimizing degeneration due to grid to plate capac-' ity of the electron discharge device. It is another object of my invention to provide an ultra high frequencyreceiving apparatus of the heterodyne type including an improved arrangement for minimizing degeneration loading effects which may occur at both radio frequencies and intermediate frequencies.

The input conductance of the electron discharge device employed in a converter which is 9 claims, (01. 250-20) to ultra high frequency I receivers of the heterodyne type, and particularly,

due to transit time loading effects decreases with increased plate voltage, and a high plate voltageis, therefore, desirable provided that adequate oscillator voltage, and adequate converter emission and safe plate dissipation are available. Un-

der optimum conditions of operation, the amplitude of the oscillator voltage is critical and therefore it'is desirable to'maintain the voltage of the local oscillator output uniform throughout a substantial range of frequencies, and it is a further object of my invention to provide an improved oscillation generator for providing uniform and adequate voltage at ultra high frequencies with a minimum loading effect on the converter circuit. a

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself,- however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a diagrammatic illustration of an ultra high frequency converter embodying my invention; Figs. 2 and 4' are bridge circuit diagrams, and Figs. 3 and 5 are curves illustrating the operating characteristics of the con.- verter shown in Fig. 1; andFig. 6 is a'diagrammatic illustration of a modified form of my invention.

Briefly, the frequency conversion apparatus illustrated in the drawings comprises a receiver of the heterodyne type including -a three-electrode electron discharge device connected as a converter and asecond three-electrode electron discharge device connected :as a local oscillator. Radio frequency signals are-supplied to the converter device through a tuned grid circuit, and the oscillations produced by the local generator are also coupled to the grid of the converter.

' The oscillator isprovided with a tuned grid circuit which is grounded, and the-electron discharge device of the oscillator has its plate grounded at radio frequencies. An inductance is provided in the cathode lead of the oscillator discharge device and has a critical value at which the grid current in the oscillator is uniform over the tuning range of the grid'circuit. The input conductance of the converter device, which is.

anode l4, and a cathode I5.

Q plate capacity in a three electrode device.

j sistance. ratio of input resistance to the sum of the input I resistance and the equivalent noise resistance so that the limit of sensitivity is more closely i attained.

tance in series resonance between cathode and ground at the intermediate frequency.

The conversion apparatus shown in Fig. l'comprises a converter land a local oscillator ll. The converter includes an electron discharge device l2 having a control electrode or grid l3, an

jline including a shielding tube It and a conducting lead IS. The bar 16 is mounted in an' adjustable conducting clamp 20 and the circuit is tuned by adjusting the position of the bar to change the length of the bar from the grid to the grounded chassis. The output circuit of the converter is connected between the anode l4 and ground, a suitable source of direct current, such as a battery 22, being provided. The output circuit includes an intermediate frequency transformer 23 and a radio frequency choke 24. The primary winding of the transformer 23 is connected to ground for alternating currents by a suitable condenser 25 to by-pass the battery 22. The load c rcuit is completed by a grid biasing resistance 26 and a radio frequency choke 21 connected in series between the cathode l5 and j ground. A condenser 28 is connected across resistance 26 between the choke coil 21 and ground.

A condenser 30 is connected in the cathode lead between the cathode I5 and ground, and a condenser 3| is connected between the anode I 4 and ground for reasons to be explained below. The

cathode lead and the lead connecting the anode to ground through the condenser 3! are preferably made as short as possible.

At ultra high frequencies, degenerative loading of the grid of the electron discharge device of the 3 converter may occur due to the self-inductance of the cathode lead and also due to the grid to This degeneration increases the input conductance and consequently reduces the effective signal potential at the grid. High input resistance is essential if maximum signal strength is to be attained at the control electrode of the converter,

' and it is therefore necessary to reduce or limit f the degenerative loading of the grid circuit. The

noise intensity may be expressed in terms of an equivalent resistance ,in series with the control electrode. The output signal'depend's upon the input resistance while the noise depends upon the sum of theinput resistance and the noise re- I-Iigh input resistance means a higher In order to minimize or prevent grid loading due to degeneration, the condensers 30 and iii are. selected to be of such sizes that they, together with the grid to cathode and grid to anode capacities of the device it, constitute the four arms of a bridge, as indicated in Fig. 2', so that the bridge may be substantially in balance at frequencies within the range of radio frequencies to be impressed upon the grid 13. In Fig. 2 the grid to cathode capacity is-indicated at 32 and the grid to anode capacity at 33. In representing I the electron discharge device 12 and the capacity 1 bridge in Fig. 2, the conventional equivalent gen- :erator elements have been employed, the plate resistance of the .tube being indicated as T and A tuned circuit the equivalent generator as [Leg- Although the numerals 30 and 3| have been employed in Fig. 2 to indicate the capacity between ground and the cathode and plate, respectively, it should 'be understood that these capacities in the bridge include all the capacity between the cathode and ground and plate and ground. For example, the cathode to ground capacity includes the capacity between the cathode l5 and its heater 36, if rounded, and, furthermore, the inductance of the cathode circuit to ground must also be taken into consideration as it may modify the effective capacity from cathode to ground. In Fig. 3 the converter output voltage is plotted against the represents the signal voltage at the output and the lower curve the noise voltage at the output. As shown by these curves, the bridge may be balancedby adjusting the variable condenser 38, and the signal to noise ratio is greatly increased over the peak portion of the signal curve, the peak of the curve occurring at bridge balance. It will readily be apparent that over the peaked portion of the curve the strength of the signal in the output has been increased materially, whereas the noise remains substantially constant. The signal to noise ratio is, therefore, substantially increased.

A further extension of the frequency range of the converter may be realized by selecting the values of the condensers 30 and 3l' so that they may be tuned to series resonance with the cathode and anode lead inductances respectively. Thus the bridge may be balanced by producing substantially zero impedance in two arms so that the anode and the cathode are both short-circuited througha low impedance to ground potential. Preferably the cathode and anode leads are tuned with their respective condensers to series resonance at the same frequency within R. F. operating range. The bridge circuit for this operation is indicated in Fig. 4 where the cathode and anode lead inductances are indicated at 38 and 39 respectively. In Fig. 4, since the condenser 30 and cathode lead inductance 38 are tuned to resonance at a frequency within the range of frequencies under consideration, the capacity represented by the condenser 30 in the bridge is the actual capacity of the condenser 30 and is not intended to include other capacities such as the cathode heater capacity. These other capacities are ineffective in the series resonant circuit.

From Figs. 2 and 4 it will now be apparent that a neutralizing bridge may be arranged such that at lower frequencies, substantial bridge balance will be obtained which will efiectively neutralize loading of the grid due to degeneration, and at higher frequencies the degeneration may be effectively eliminated by the connection of the r plate and cathode to ground through the series resonant connection.

It is also necessary to prevent degeneration atthe intermediate frequency, however, the bridge circuits indicated in Figs. 2 and 4 are unbalanced for frequencies of the order of the intermediate frequency. Degeneration at intermediate frequency may be prevented by tuning the inductance 21 and condenser 28 in series resonance between the cathode and ground at the 7 intermediate frequency. The reactance values of the inductance 21 and condenser 28 are made sufficiently low that degeneration is prevented over a wide range of frequencies in the neighborhood of the intermediate frequency. f u

The value of the resistor 26 is selected so that cathode to ground capacity. The upper curve the "resulting bias on the grid l3 maintains the grid about at the cut-off voltage of the device I2. The oscillator l l is coupled to the grid [3 through a condenser 40, and the peak voltage of the oscillator H is preferably about equal to the bias voltage of the grid !3. The oscillator frequency may be either above or below the signal frequency. However, it has been found that it is preferable to operate the oscillator at a frequency I below the signalfrequency and that the stability of the oscillator is increased thereby, and adequate excitation of the converter is assured. The explanation'of the advantage to be gained in this manner lies in the fact that. the signal input is capacitive at the higher oscillator frequency, and only a portion of the oscillator voltage appears on the grid [3, as determined by the capacity divider including the condenser 40 and the capacitive signal input circuit; Furthermore, the signal voltage tends to be greater at the oscillator than at the grid I3 because the oscillator circuit is inductive at the signal frequency and a partial series resonance tends to occur. If the oscillator is operated at a frequency lower than that of the signal, more oscillator voltage may be made to appear on the grid l3 than exists at the oscillator, and, also, the coupling of the signal to the oscillator is reduced. The result of operating the oscillator at a lower frequency than the signal frequency is a definite improvement in signal to noise ratio; in some cases the ratio may be doubled.

In order to decrease the input conductance due to the transit time of electrons through the discharge device ll of the converter, it is desirable to employ a high voltage on the plate l4. However, when a high voltage is employed, it is also desirable that the oscillator voltage impressed upon the grid l3 be maintained at a definite value which is substantially uniform over the range of frequencies within which the oscillator is required to operate. In order that the oscillator voltage may be maintained at a substantially fixed level the plate or anode circuit of the "oscillator is grounded at radio frequencies.

Referring again to the drawing, the oscillator l I shown in Fig. 1 comprises an electron discharge device 42 having an anode 43, a control electrode or grid 44, and a cathode 45. Between the control electrode 44 and ground is connected a series tuned circuit including atransmission line or bar conductor 46 connected to the grid 44, and condensers 48 and 49 in parallel connected between the other end of the bar 46 and ground. The'bar 46 ispreferably mounted parallel to a grounded chassis or shielding plate of the oscillator and is spaced a short distance therefrom. A control electrode biasing resistance or grid leak 4'! is connected between the bar 45 and ground near the end to which the condensers 48 and 49 are connected. The condenser 49 is variable and is employed to tune the resonant circuit. Theefi'ective length of the transmission line 46 is in creased by the capacity of the condensers 48 and 49, the effective length being tuned to a quarter wavelength at the desired frequency so that the total voltage of the tuned circuit appears at the end of the bar 46 adjacent the grid 44, as indicated by the dotted voltage curve'4l, the minimum voltage appearing at the bar adjacent its connection with the resistance 41. The .condenser 48 limits the tuning rangeof the condenser 49 so that the condenser 49 will not have I rapid tuning at the minimum and of its range.

The anode 43 is connected at radio frequencies to ground through a condenser 50. The direct 'cur rent power supply, such as a battery 5|, is connected to the anode 43 through a radio frequency choke coil 52. The device 42 is providedwith a filament heater 53 which is supplied with current from a battery or other suitable source 54 through a radio frequency choke 55, the current returning through an inductance 56 in the cathode lead of the device 42; a by-pass condenser 51 "is-provided across the two leads of the heater 53 and a condenser 58 is provided to by-pass the battery 54 at radio frequencies. The grid 44 at which the full voltage of the oscillator appears is coupled to the grid l3 of the converter device [2 through the condenser 46. Since the plate 43 is grounded at radio frequencies, the voltage appearing at the grid 44 may be adjusted and main-' tained at a predetermined level. V

It has been found that the size of the inductance coil 56 in the cathode circuit of the device 42' is critical and that by selecting the proper value of this inductance the voltage of the oscillator may be made substantially uniform at all frequencies within the tuning range. In Fig. 5 the grid current of the device 42 has been plotted against frequency for three values of the inductance 56. The two vertical lines in Fig. 5 represent the lower and upper limits of the fre-' quency range of the oscillator; the dotted line 60 represents the grid current curve'when the correct value of the inductance 56 is employed; the dotted curve 6| represents the grid current when the value of the inductance 56 is too small; and the curve 62, which is a similar curve, showing the current when the value of the inductance 56 is too large. The correct value of the inductance 56, as indicated by the curve 60, makes it possible to tune the oscillator ll over its entire range of frequencies without any substantial change in the adequate and uniform voltage supplied to the converter.- Values of the inductance either too large or too small may cause the oscillations to stop, or on the other hand, may cause the oscillator to superregenerate. In either case uniformity of operation is obviously lost.

be reduced until it is' negligible by close spacing between the grid line 46 and the shield or chassis of the oscillator. By arranging the oscillator so that the grid line 46 is tuned in series with the condenser 49, it is possible-to employ a larger: condenser which is mechanically preferable and increases the stability of operation.

'During the operation of the converter the oscillator is tuned solely by adjustment of the variable condenser 49. The value of the inductance 56, once selected, is fixed so that the oscillator will operate at all frequencies within its range of frequencies with the grid current following the curve 60 of Fig. 5.

A modified form ofthe converter embodying the'invention is shown in Fig. 6. Corresponding elements have been designated by the same numerals as applied to the converter shown in Fig. l. The tuned grid circuit comprises a wound inductance element connected in parallel with avariable condenser 66 between the grid l3' and ground. An input line filis'connected across a portion of the inductance 65,. a grounded concentric shield 6.8 being provided about the line' 61; The plate circuit of the device I2 includesa resistance :69 connected between the primary;

winding;goffthettransformen:23; andgtherzpositiyes converter: The selected $31116: ofqthetcondenseisr side;;of.-tliebatteryt 22; thecgrounded condenser 301is;therefora a,compromise;itibeing threeion '25i' g 1 d to theljunctionl-of the primary four :times the valuerofi the -cathode=-to.-grid-,cai-' winding and' the resistance--:-69; The circuit "for; pacity 32: r supplyingy 'the heater--36 islshown as :including; 5: Although I' have illustrated my invention-gas a-battery 710;; connected to the; heaterithrough"; embodied in a";superheterodyne-receiver, others radiofrequency chokes it-and 12,- by-passcon; applications will readily beapparent to-those densers; 13-... and; Hi: being zplfoilided across; the? skilled in vvthe art: I-do not; therefore,-.desire-my;-

heater-36yand1 thebattery Ill; respectively; The; inventionto be limitedto the particular-circuits negative terminal of the battery; is" grounded." l arrangement-shown and described,;and I intend: The:converter;of Fig.1 fivincludes the same-bridgein the appended claims.--to cover all modifications. ing: circuit has that of,;.Fig;;;,1,1;and' the anode-14" within the true spirit and-,scopeofmyinvention; and:.cathode are connectedrlto ground by series :7 .What I-claim as new: and desire to, secure byi resonantzcircuits.at:irequencieszimthe high-free LetterslPatentof theeUnited States is:

quencyzflportiomof the range, .ofz;the converter :in 15; :In ish fr q e yyr c v s app r s-an the gsameemanner as in: the: converter of .Fig: 1; electron discharge device l'iaving; an anode: and} Thegridmircuitis.tunedibyadjusting the econ-a a----cathode andsa control electrode, meansion denser 66. The oscillator I l;-.may; be-:c oupled;to.; impressing on said control electrode oscillations; the g rid 13 through the condenser 40 in the same within a-predeterminedrange of ultra-high 'fres,-

manner: as in .Fig. 1.7,. I ;quencies, an output. circuit connected-betweenn. :Itiwill be evident that my invention provides said anode and said"; cathode, a capacitor, con-g aifrequencyconversion apparatuswhich is struc: nectingrsaidanode ;to vground-, a. capacitor con-- turally simple and employs-three electrode elecnectingsaid cathode to ground, and a: bridge: tron discharge devices at ultra high frequencies. circuit, the first two :arms-of which-comprise--re-;- Bygway of'illustration'only, and notby limita-, vspectively thecathode tougroundcapacity; and-s tion, there are listed below values of circuit the-anode to ground. capacityof .said device-ands; constants which have been found to be suitable the second two :arms oflwhich comprise: respect forsthe circuit of Fig, 1 whenemployed in apartively the. cathode to control :electrode capacityy ticular superheterodyneVfrequencyconverter opandrther-anode to control electrode capacity of; eratingtat aradio? frequency of 400 megacycles said device,- said:bridge being, in balance forte-e; and with-theoscillatorgtuned to either 358 megafrequency within the lower {portion of ;said range A, cycles or 442 megacycles :to provide :an intermeof frequencies whereby degeneration at said:lo-wer-- diate frequency of 42 megacycles. The devices 2 frequency is minimized, said first two arms of said and-42 were type' 955' Acorn triodes. The bars bridges-also. including inductanceand having their; lfi-gandgdfi-were one fourth inch square brass rods respective capacitorsitunedqin' vseriesswith their approximately four inches long which had been respective inductances to resonanceatairequency silver: platedaand were spaced one eighth of an in the upper portion of said rangeof frequencies. inch: from thegroundedpartition l1. 1 whereby degenerationatrsaid.upperfrequency-isv r v v minimized.

, Converter 10 40 2. In an ultra-high frequency-energy receiving Condenser25='1000' ,u farads apparatus; an electromdischarge. device: having; Condenser 28:220 t farads an anode and a cathodeandtaacontrol electrode,. Condenser39=3-30 rfarads- V 1 g Y means :for impressing on said-control electrode- Condenser 31:2 farads" l voltage-waves within axpredetermined range of; Choke 23:0.2 uhenryl 1 frequencies, means including a condenser forcons Chol:ev 2 'l,' .064 ;r henry' r nectingrsaidanodeto ground, means includingwa. Resistance 26:300-3000 ohms r condensersfor connecting'said cathodeto ground I Transformer 23:42 mc'. I. F.'transformer each of said connecting means including an in-l Coupling'condenser 40:70.2 [.t/L farad, oscillator. ati ductance tuned in series with its respective cone 358 -mc., 2 p41. farads, oscillator at 442 mc.- denser to substantial resonance within said range .1

- Oscillator 11 of'frequencies whereby degeneration atfrequene r v cies. within said range, of frequencies is mini-. Condenser 48=5,;t .'farads r v mized, andan output circuit connected between Condenser4$=3 to 12, ,u ufarads. r 1 saidranode and said cathode. v

99 3 5- mk ar V 3;: Inan ultra highfrequency energy receiving Condenser 51:10) M farads apparatus, an electron-discharge device having.

C 53:250 MM famds I an anode and vat-cathode and a control electrode, Choke 7 a t at 400 m means for impressingon said control electrode. Choke .55=resonant at, 400 mo. 7 l l voltage waves Within a predetermined range of F h frequencies,meansrincluding capacity and induct-r Resistance 4 l=8500 0h ms@- tance in series. for connecting said anodev to,

-Pif ab1y th capacity 32 should:b-e;smauer 1 ground,.means;includingcapacityand-inductance: th th t of th condenser 39 in Order t t b.v in. :seriesrich-connecting said;cathode to1 ground;

t -many ,11 the tuned grid circuit voltage 1 each of said connecting means beingseries tuned pressed upon t grid appearsbetween t grid' 65 to-sulestantial :resonance for frequencies within and the cathode; Obviously; the bridge' can be: Said n e eq11encies;- said two, connecting; b a i t relativelyr large vames of means. constituting two arms of'a bridge the densers so and 3|; However, the condenser 3| Y er wo arms of- Which comprise,- p. t r adds-capacity to the anode circuit and therefore thexc trol electrode to :anode. capac ty andrthes the gain for-"a; given bandwidth would be reduced cathqdeto' control electrode capacity ofrysaidsdee by increasing this capacity As: a result, the-J. 'vice whereby degener-ationatfrequenciesawithinsv signal to noise ratio would be reduced underg'cone said range of frequenciesis minimized,:and-l,-an

ditions such that appreciable-noise existsrin'firstw output :circuit connectedtosaid anoder electron dischargetubes in theintermediate fre-ir ,4; In:angultraflnigh.frequency;energy;receiving;

quencies;circuittconnectedztoithe -;out-put::ohtheg "apparatus amelectrorr discharge deviceahaving;

' an anode and a cathode and a control electrode,

comprise respectively the control electrode to anode capacity and the cathode to control electrode capacity of said device, said bridge being balanced at a predetermined frequency within said range whereby degeneration at said frequency is minimized, and an output circuit connected to said anode.

5. In an ultra high frequency energy receiving apparatus, an electron discharge device have an anode and a cathode and a control electrode, means for impressing on said control electrode. voltage waves within a predetermined range of frequencies, means including a condenser and inductance for connecting said anode to ground, means including a condenser and inductance in series for connecting said cathode to ground, said condensers constituting two arms of a bridge the other two arms of which comprise respectively the control electrode to anode capacity and the cathode to control electrode capacity of said device, said bridge being balanced at a predetermined frequency within said range whereby degeneration at said frequency is minimized, said condensers and inductances of said connecting means being respectively series tuned to resonance at a. higher predetermined frequency within said range whereby said anode and said cathode are directly connected to ground at said higher frequency, and an output circuit connected to said anode and cathode.

6. In an ultra high frequency heterodyne receiver, an electron discharge device having an anode and a cathode and a control electrode, means for impressing on said control electrode signal oscillations within a predetermined range of frequencies, means for impressing on said control electrode local oscillations of a frequency different from that of the signal. oscillations, an output circuit, for said device including means tuned to the difference in frequency between said signal oscillations and said local oscillations,- mean including a condenser for connecting said anode .to ground, and means including a condenser for connecting said cathode to ground, each of said connecting means including an inductance tuned in series with its respective condenser to substantial resonance within said range of frequencies whereby degeneration at frequencies within said range of frequencies'is minimized.

7. In an ultra high frequency heterodyne receiver, an electron discharge device having an anode and a cathode and a control electrode, means for impressing on said control electrode signal oscillations within a predetermined range of frequencies, means for impressing on said control electrode local oscillations of a frequency different from that of the signal oscillations, an output circuit for said device including means tuned to the difference in frequency between said signal oscillations and said local oscillations, means including a condenser for connecting said cathode to ground, and means including a condenser for connecting said anode to ground, each of said connecting means including an inductance tuned to substantial resonance with its re-, spective condenser at frequencies in the upper portion of said range of'frequencies for minimizing degeneration at said upper frequencies, said condensers constituting two arms of a bridge the other two arms of which comprise the control electrod to anode-capacity and the cathode to control electrode capacity of said device, said bridge being balanced at frequencies in the lower portion of said range of frequencies whereby degeneration at said lower frequencies is minimized.

8. In an ultra high frequency heterodyne receiver, an electron discharge device having an anode and a cathode and a control electrode, means for impressing on said control electrode signal oscillations within a predetermined range of frequencies, means for impressing on said control electrode loca1 oscillations of a frequency different from that of the signal oscillations, an output circuit for said device including means tuned to the difference in frequency between said signal oscillations and said local oscillations, means including a condenser for connecting said anode to ground, means including a condenser for con-' necting said cathode to ground, each of said connecting means including an inductance tuned in series with its respective condenser to substantial resonance within said range of frequencies, and means including a second condenser and a second inductance in series therewith and tuned at said difference frequency for connecting said cathode to ground at said difference frequency for minimizing degeneration at said difference frequency.

9. In an ultra high frequency heterodyne receiver, a first electron discharge device having an anode and a cathode and a control electrode, means for impressing on said control electrode signal oscillations within a predetermined range of frequencies, means including a local oscillator for impressing on said control electrode oscillations at a frequency different from that of the signal; an output circuit for said first, device including means tuned to the difference in frequency between said signal oscillations and said local oscillations, means including a condenser for connecting said anode to ground, means including a condenser for connecting said cathode to ground, each of said connecting means including an inductance tuned in series with its 4 respective condenser .to substantial resonance within said range of frequencies whereby degeneration at frequencies within said range of frequencies is minimized, said local oscillator comprising a second electron discharge device having an anode and a cathode and a control electrode, means for tuning the control electrode circuit of said second device to resonance at said different frequency, means connecting the anode of said second device .to ground at said different frequency, and a condenser for coupling the control electrode of said second device and the control electrode of said first device.

GEORGE W. FYLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Worchester Jan. 19, 1943 

